Magnetic recording media comprising a non-magnetic support having thereon a magnetic layer comprising a binder having dispersed therein a magnetic powder, such as ferromagnetic iron oxide powder, Co-doped ferromagnetic iron oxide powder, CrO.sub.2 powder, and a ferromagnetic alloy powder, have widespread applications as video tape, audio tape, and magnetic discs.
Short wave recording has recently been introduced to meet the demand for an increased recording density. For example, the recording wavelength for 8 mm-video tape has reached 0.54 m.mu.. With this tendency, there has arisen a problem of so-called thickness loss on reproduction, that is, reproduction output is reduced as a function of increasing magnetic layer thickness.
In order to cope with this problem encountered in short wave recording, magnetic recording media using a thin film of a ferromagnetic metal have been put to practical use which have a very small thickness due to use of formation methods such as vacuum deposition techniques. Such metal-deposited recording media suffer little thickness loss and attain a very high reproduction output. However, production of such magnetic metal thin film-deposited recording media by vacuum evaporation of a metal on a non-magnetic support is less suited to mass production as compared with so-called coated type magnetic recording media formed by the conventional coating techniques involving dispersions of ferromagnetic powders in a binder system. In addition, the metallic film is less reliable for long-term use because of susceptibility to air oxidation. Therefore, it has been attempted to instead reduce the thickness of a magnetic layer formed by various manipulations of the conventional coating technique to thereby increase reproduction output. However, as the thickness of a magnetic layer is decreased to about 2 .mu.m or less, the surface properties of a support are apt to strongly influence the surface properties of the magnetic layer resulting in deterioration of electromagnetic characteristics.
In order to reduce the thickness of a magnetic layer to minimize thickness loss of magnetic properties thereby achieving a high output while excluding the adverse influences of a support surface, it has been proposed to provide a thick non-magnetic layer between a non-magnetic support and a thin magnetic layer. For example, U.S. Pat. No. 2,819,186 discloses a magnetic recording medium comprising a support having thereon a hard and brittle magnetic layer having a magnetic substance content of 85% by weight or more and a thickness of not more than 0.25 mil as an upper layer and a soft and flexible non-magnetic lower layer having a higher thickness than the upper magnetic layer. JP-A-62-154225 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a magnetic recording medium having a magnetic layer of 0.5 .mu.m or less in thickness with a subbing layer provided between the magnetic layer and the support wherein the subbing layer contains carbon black as a conductive fine powder and has a thickness greater than the magnetic layer so as to prevent surface resistance of the magnetic layer from increasing. JP-A-62-222427 discloses a magnetic recording medium comprising a support having thereon a subbing layer containing an abrasive having an average particle size of from 0.5 to 3 .mu.m and a magnetic layer having a thickness of 1 .mu.m or less and containing a ferromagnetic powder, in this order, in which a part of the abrasive in the subbing layer projects through the magnetic layer so as to serve for cleaning of a magnetic head. JP-A-63-191315 discloses a magnetic recording medium having a lower non-magnetic layer and an upper magnetic layer formed by simultaneous coating, in which the lower layer has a thickness of 0.5 .mu.m or more and contains no polyisocyanate.
However, a magnetic coating composition must be diluted with a relatively large quantity of a solvent so as to be coated to a dry thickness of not more than 1.0 m. Such a diluted coating composition is susceptible to agglomeration. Further, orientation of a ferromagnetic powder is apt to be disturbed during drying due to evaporation of the large quantity of organic solvent. When the medium has a non-continuous form, for example, if it is a magnetic disc, adequate performance properties may still be obtainable to some extent even in using such a thin magnetic coating composition. However, with respect to those media having a continuous form, such as magnetic tapes, although the purpose of thickness reduction is accomplished, it is difficult to obtain sufficient electromagnetic characteristics because of deteriorated orientation and deteriorated surface properties. In addition many voids are produced during drying, resulting in poor film strength in running. If the amount of the diluting organic solvent is decreased in order to improve orientation properties and to minimize voids, the coating stability would be deteriorated, leading to formation of many pinholes and an increased production of defective media.
Further, although a thick non-magnetic layer formed beneath a magnetic layer eliminates the influences of the surface roughness of the support, the problem of wearability against a recording head or durability is left unmitigated. The poor wearability or durability of conventional magnetic recording media having a non-magnetic lower layer appears attributable to curing of the lower layer comprising a thermosetting resin as a binder for reasons that the magnetic layer formed thereon is brought into contact with a head or other members without cushioning and that the magnetic recording media having such a cured lower layer lacks the desired flexibility.
This problem might be resolved by using a non-curing (thermoplastic) resin as a binder in the lower layer. However, when a magnetic layer is coated on a dried lower layer containing such a non-curing resin, as in the conventional technique, the lower layer is swollen with the organic solvent of the magnetic coating composition, giving undesired influences, such as turbulence of the magnetic coating composition, leading to impairment of the surface properties of the magnetic layer and deterioration of electromagnetic characteristics.